Broadband reflectors, concentrated solar power systems, and methods of using the same

ABSTRACT

Broadband reflectors include a UV-reflective multilayer optical film and a VIS/IR-reflective layer. In various embodiments, the VIS/IR reflective layer may be a reflective metal layer or a multilayer optical film. Concentrated solar power systems and methods of harnessing solar energy using the broadband reflectors and optionally comprising a celestial tracking mechanism are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/142,910, now U.S. Pat. No. 9,523,516, issued Dec. 20, 2016, which isa national stage filing under 35 U.S.C. 371 of PCT/US2009/068944, filedDec. 21, 2009, which claims priority to U.S. Provisional PatentApplication No. 61/141338, filed Dec. 30, 2008, and U.S. ProvisionalPatent Application No. 61/178123, filed May 14, 2009, the disclosures ofwhich are incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present disclosure relates to reflectors suitable for reflecting abroad range of electromagnetic radiation, concentrated solar powersystems including the reflectors, and methods of using the same.

BACKGROUND

Concentrated solar power (CSP, also known as “concentrating solarpower”) technology uses sunlight directed at heat transfer fluids thatheat up and whose thermal energy is then transferred (e.g., for heating)or turned into electrical power (e.g., by use of a turbine generator).Conventional CSP reflectors are made with silver coated glass, whichcurrently reflect approximately 94 percent of the solar spectrum. SuchCSP reflectors are relatively expensive, heavy, and fragile.

CSP systems typically use lenses or reflectors and tracking systems tofocus a large area of sunlight into a small beam. The concentratedsunlight is then used as a heat source for a conventional power plant(e.g., a steam driven turbine generator). A wide range of concentratingtechnologies exists; the most developed are the solar trough, parabolicdish and solar power tower.

Solar troughs are the most widely deployed and the most cost-effectiveCSP technology. A solar trough consists of a linear parabolic reflectorthat concentrates sunlight onto a receiver positioned along thereflector's focal line. The reflector is made to follow the sun duringthe daylight hours by tracking along a single axis.

A parabolic dish system consists of a stand-alone parabolic reflectorthat concentrates sunlight onto a receiver positioned at the reflector'sfocal point. The reflector tracks the sun along two axes. Parabolic dishsystems give the highest efficiency among CSP technologies. Power towersare less advanced than trough systems but offer higher solarconcentration ratios (e.g., in excess of 1000 times more) and betterenergy storage capability.

SUMMARY

CSP utility plants are being installed in high solar irradiationclimates around the world with investment projections to exceed $200million/year in the next 5 years. Advantageously, broadband reflectorsaccording to the present disclosure may be fabricated with at least oneof lower weight, lower cost, and/or improved power collection capabilityas compared to conventional reflectors used in CSP.

In one aspect, the present disclosure provides a broadband reflectorcomprising: an ultraviolet-reflective (UV-reflective) multilayer opticalfilm having a first major surface and comprising a UV-reflective opticallayer stack, wherein the UV-reflective optical layer stack comprisesfirst optical layers and second optical layers, wherein at least aportion of the first optical layers and at least a portion of the secondoptical layers are in intimate contact and have different refractiveindexes; and a visible/infrared-reflective (VIS/IR-reflective) metallayer disposed on at least a portion of the first major surface.

In another aspect, the present disclosure provides a broadband reflectorcomprising: a UV-reflective multilayer optical film having a first majorsurface and comprising a UV-reflective optical layer stack, wherein theUV-reflective optical layer stack comprises first optical layers andsecond optical layers, wherein at least a portion of the first opticallayers and at least a portion of the second optical layers are inintimate contact and have different refractive indexes, and wherein theUV-reflective optical layer stack is reflective to UV-light; aVIS/IR-reflective multilayer optical film comprising a VIS/IR-reflectiveoptical layer stack, wherein the VIS/IR-reflective optical layer stackcomprises third optical layers and fourth optical layers, wherein atleast a portion of the third optical layers and at least a portion ofthe fourth optical layers are in intimate contact and have differentrefractive indexes, and wherein the VIS/IR-reflective multilayer opticalfilm is reflective to VIS/IR-light; and a UV-absorbing layer disposedbetween the UV-reflective multilayer optical film and theVIS/IR-reflective multilayer optical film. In some embodiments, thefirst optical layers and second optical layers and/or the third opticallayers and fourth optical layers respectively comprise a polyethyleneterephthalate and a THV, a polyethylene terephthalate and an OTP, a PENand a THV, a PEN and an OTP, a PEN and a PMMA, a polyethyleneterephthalate and a coPMMA, a PEN and a coPMMA layer pairs, a coPEN anda PMMA layer pairs, a coPEN and an OTP, a coPEN and a THV, a sPS and anOTP, a sPS and a THV, a PMMA and a THV, a COC and a THV, or an EVA and aTHV layer pairs.

In some embodiments, the UV-reflective multilayer optical film furthercomprises a tie layer that forms the first major surface of theUV-reflective multilayer optical film. In some of those embodiments, thetie layer comprises an inorganic tie layer. In some embodiments, theinorganic tie layer comprises titanium dioxide or aluminum oxide. Insome embodiments, the VIS/IR-reflective metal layer comprises at leastone of silver, copper, stainless steel, or aluminum (i.e., comprisessilver, copper, stainless steel, aluminum, or any combination thereof).

In another aspect, the present disclosure provides a broadband reflectorcomprising: a UV-reflective multilayer optical film having a first majorsurface and comprising a UV-reflective optical layer stack, wherein theUV-reflective optical layer stack comprises first optical layers andsecond optical layers, wherein at least a portion of the first opticallayers and at least a portion of the second optical layers are inintimate contact and have different refractive indexes, and wherein theUV-reflective optical layer stack is reflective to UV-light; aVIS/IR-reflective multilayer optical film comprising a VIS/IR-reflectiveoptical layer stack, wherein the VIS/IR-reflective optical layer stackcomprises third optical layers and fourth optical layers, wherein atleast a portion of the third optical layers and at least a portion ofthe fourth optical layers are in intimate contact and have differentrefractive indexes, and wherein the VIS/IR-reflective multilayer opticalfilm is reflective to VIS/IR-light; and a UV-absorbing layer disposedbetween the UV-reflective multilayer optical film and theVIS/IR-reflective multilayer optical film.

In some embodiments, the broadband reflector has an average lightreflectivity of at least 95 percent over a wavelength range of 350 to400 nanometers. In some embodiments, the broadband reflector has anaverage light reflectivity of at least 90 percent over a wavelengthrange of from 300 to 2494 nanometers.

In some embodiments, the UV-reflective multilayer optical film furthercomprises a second major surface opposite the first major surface, andwherein the UV-reflective multilayer optical film further comprises anabrasion resistant layer that forms the second major surface of theUV-reflective multilayer optical film. In some of those embodiments, theabrasion resistant layer comprises: an antisoiling component selectedfrom the group consisting of fluoropolymers, silicone polymers, titaniumdioxide particles, polyhedral oligomeric silsesquioxanes, andcombinations thereof. In some embodiments, the abrasion resistant layercomprises a conductive filler; for example, to facilitate static chargedissipation.

In some embodiments, the broadband reflector is thermoformable. In someembodiments, the broadband reflector has a parabolic or curved surface.

In yet another aspect, the present disclosure provides a concentratedsolar power system comprising: at least one broadband reflectoraccording to the present disclosure and capable of being aligned todirect solar radiation onto a hollow receiver; and a heat transfer fluidpartially disposed within the hollow receiver.

In some embodiments, the concentrated solar power system furthercomprises an electrical generator in fluid communication with the hollowreceiver.

In some embodiments, the concentrated solar power system furthercomprises a celestial tracking mechanism for the at least one broadbandreflector. The at least one broadband reflector may, for example, bepivotally mounted on a frame. In some embodiments, both the hollowreceiver and the at least one broadband reflector are pivotally mountedon a frame. The pivotally mounted components may pivot, for example, inone direction or in two directions. In some embodiments, the hollowreceiver is stationary.

In yet another aspect, the present disclosure provides a method ofharnessing solar energy, the method comprising reflecting solarradiation using at least one broadband reflector according to thepresent disclosure onto a hollow receiver containing a heat transferfluid to provide a heated heat transfer fluid.

In some embodiments, the method further comprises thermally heating atleast a portion of a building with heat given off from the heated heattransfer fluid. In some embodiments, the method further comprisesgenerating electrical power using the heated heat transfer fluid.

In yet another aspect, the present disclosure provides a broadbandreflector comprising:

a UV-reflective multilayer optical film having a first major surface andcomprising a UV-reflective optical layer stack, wherein theUV-reflective optical layer stack comprises first optical layers andsecond optical layers, wherein at least a portion of the first opticallayers and at least a portion of the second optical layers are inintimate contact and have different refractive indexes;

an adhesive layer disposed on at least a portion of the first majorsurface; and

a VIS/IR-reflective metallic substrate disposed on at least a portion ofthe adhesive layer.

In some embodiments, the adhesive layer comprises a pressure-sensitiveadhesive layer.

In this application:

“light” refers to electromagnetic radiation, whether visible to theunaided human eye or not;

“optical layer” means having a layer of a material having a thickness ina range of about one quarter of a wavelength or wavelengths of light tobe reflected (e.g., optical layer in the context of a UV-reflectiveoptical layer stack means a layer of material having a thickness ofabout one quarter of a wavelength of UV-light, while optical layer inthe context of a VIS/IR-reflective optical layer stack means a layer ofmaterial having a thickness of about one quarter of a wavelength ofVIS/IR-light;

“polymer” refers to a macromolecular compound consisting essentially ofone or more repeated monomeric units, or a mixture of macromolecularcompounds that consist essentially of one or more like repeatedmonomeric units;

“UV-absorber” means a material capable of absorbing or blockingelectromagnetic radiation at wavelengths less than 380 nanometers (nm),while remaining substantially transparent at wavelengths greater than400 nm;

“UV-light” means electromagnetic radiation having a wavelength in arange of from greater than 350 to 400 nm;

“UV-reflective” means substantially reflective to UV-light (for example,at least 30, 40, 50, 60, 70, 80, 90, or 95 percent reflective to atleast a portion of UV-light at a 90 degree angle of incidence);

“UV-absorbing” means substantially absorbing at normal incidence (90degree angle of incidence) of light at least some wavelengths in a rangeof from 300 to 400 nanometers (for example, at least 30, 40, 50, 60, 70,80, 90, or 95 percent absorbing of light at wavelengths in a range offrom 300 to 400 nm);

“VIS/IR-light” means electromagnetic radiation having a wavelength in arange of from greater than 400 to 2494 nm; and

VIS/IR-reflective” means substantially reflective to VIS/IR-light (forexample, at least 30, 40, 50, 60, 70, 80, 90, or 95 percent reflectiveat normal incidence to light at wavelengths in a range of from greaterthan 400 to 2494 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1A is a schematic side view of a broadband reflector 100 accordingto one embodiment of the present disclosure;

FIG. 1B is a schematic side view of a UV-reflective optical layer stack140 included in broadband reflector 100;

FIG. 2A is a schematic side view of a broadband reflector 200 accordingto another embodiment of the present disclosure;

FIG. 2B is a schematic side view of a VIS/IR-reflective optical layerstack 240 included in broadband reflector 200;

FIG. 3 is a schematic plan view of concentrated solar power systemaccording to one embodiment of the present disclosure;

FIG. 4 is a schematic plan view of concentrated solar power systemaccording to one embodiment of the present disclosure;

FIG. 5 is a plot of percent reflection versus wavelength for thebroadband reflector of Example 1;

FIG. 6 is a plot of percent reflection versus wavelength for thebroadband reflector of Example 2;

FIG. 7 is a schematic diagram of an embodiment of a tracker device formoving linear parabolic broadband reflectors disclosed herein mounted ina frame;

FIG. 8a is a diagram showing an embodiment of an array of hollowreceivers with louvers comprising the broadband reflectors disclosedherein, wherein the louvers are oriented to enhance capture of rays fromthe morning sun;

FIG. 8b is a diagram showing an embodiment of an array of hollowreceivers with louvers comprising the broadband reflectors disclosedherein, wherein the louvers are oriented to enhance capture of rays fromthe mid-day sun; and

FIG. 8c is a diagram showing an embodiment of an array of hollowreceivers with louvers comprising the broadband reflectors disclosedherein, wherein the louvers are oriented to enhance capture of rays fromthe evening sun.

DETAILED DESCRIPTION

Referring now to FIG. 1A, broadband reflector 100 according to oneexemplary embodiment of the present disclosure has UV-reflectivemultilayer optical film 110 with first major surface 115 andUV-reflective optical layer stack 140. VIS/IR-reflective metal layer 130is disposed on at least a portion of first major surface 115.UV-reflective multilayer optical film 110 may contain additional layerssuch as, for example, optional tie layer 120 and optional abrasiveresistant hardcoat 150. UV-reflective multilayer optical film 110reflects UV-light while VIS/IR-light that is transmitted throughUV-reflective multilayer optical film 110 is reflected byVIS/IR-reflective metal layer 130. Optional adhesive layer 125 isdisposed on metal layer 130 opposite UV-reflective multilayer opticalfilm 110.

UV-reflective optical layer stack 140 will be better understood withreference to FIG. 1B. UV-reflective optical layer stack 140 comprisesfirst optical layers 160 a, 160 b, . . . , 160 n (collectively firstoptical layers 160) in intimate contact with second optical layers 162a, 162 b, . . . , 162 n (collectively second optical layers 162). Firstoptical layers 160 and second optical layers 162 have respectiverefractive indexes that are different. Accordingly, a reflection isgenerated at each interface between the adjacent optical layers. Lightthat is not reflected at the interface between adjacent optical layerstypically passes through successive layers and is either reflected at asubsequent interface or passes through the UV-reflective optical layerstack altogether.

The normal reflectivity for a particular layer pair is primarilydependent on the optical thickness of the individual layers, whereoptical thickness is defined as the product of the actual thickness ofthe layer times its refractive index. The intensity of light reflectedfrom the optical layer stack is a function of its number of layer pairsand the differences in refractive indices of optical layers in eachlayer pair. The ratio n₁d₁/(n₁d₁+n₂d₂) (commonly termed the “f-ratio”)correlates with reflectivity of a given layer pair at a specifiedwavelength. In the f-ratio, n₁ and n₂ are the respective refractiveindexes at the specified wavelength of the first and second opticallayers in a layer pair, and d₁ and d₂ are the respective thicknesses ofthe first and second optical layers in the layer pair. By properselection of the refractive indexes, optical layer thicknesses, andf-ratio one can exercise some degree of control over the intensity offirst order reflection. For example, first order visible reflections ofviolet (400 nanometers wavelength) to red (700 nanometers wavelength)can be obtained with layer optical thicknesses between about 0.05 and0.3 nanometers. In general, deviation from an f-ratio of 0.5 results ina lesser degree of reflectivity.

The equation λ/2=n₁d₁+n₂d₂ can be used to tune the optical layers toreflect light of wavelength λ at a normal angle of incidence. At otherangles, the optical thickness of the layer pair depends on the distancetraveled through the component optical layers (which is larger than thethickness of the layers) and the indices of refraction for at least twoof the three optical axes of the optical layer. The optical layers caneach be a quarter-wavelength thick or the optical thin layers can havedifferent optical thicknesses, as long as the sum of the opticalthicknesses is half of a wavelength (or a multiple thereof). An opticalstack having more than two layer pairs can include optical layers withdifferent optical thicknesses to provide reflectivity over a range ofwavelengths. For example, an optical stack can include layer pairs thatare individually tuned to achieve optimal reflection of normallyincident light having particular wavelengths or may include a gradientof layer pair thicknesses to reflect light over a larger bandwidth.

A typical approach is to use all or mostly quarter-wave film stacks. Inthis case, control of the spectrum requires control of the layerthickness profile in the film stack. A broadband spectrum, such as onerequired to reflect visible light over a large range of angles in air,still requires a large number of layers if the layers are polymeric, dueto the relatively small index differences achievable with polymer filmscompared to inorganic films. Layer thickness profiles of such films canbe adjusted to provide for improved spectral characteristics using theaxial rod apparatus taught in U.S. Pat. No. 6,783,349 (Neavin et al.)combined with layer profile information obtained with microscopictechniques.

Desirable techniques for providing a multilayer optical film with acontrolled spectrum include:

-   -   1) The use of an axial rod heater control of the layer thickness        values of coextruded polymer layers as taught in U.S. Pat. No.        6,783,349 (Neavin et al.).    -   2) Timely layer thickness profile feedback during production        from a layer thickness measurement tool such as e.g. an atomic        force microscope (AFM), a transmission electron microscope, or a        scanning electron microscope.    -   3) Optical modeling to generate the desired layer thickness        profile.    -   4) Repeating axial rod adjustments based on the difference        between the measured layer profile and the desired layer        profile.

The basic process for layer thickness profile control involvesadjustment of axial rod zone power settings based on the difference ofthe target layer thickness profile and the measured layer profile. Theaxial rod power increase needed to adjust the layer thickness values ina given feedblock zone may first be calibrated in terms of watts of heatinput per nanometer of resulting thickness change of the layersgenerated in that heater zone. Fine control of the spectrum is possibleusing 24 axial rod zones for 275 layers. Once calibrated, the necessarypower adjustments can be calculated once given a target profile and ameasured profile. The procedure is repeated until the two profilesconverge.

The layer thickness profile (layer thickness values) of this UVreflector can be adjusted to be approximately a linear profile with thefirst (thinnest) optical layers adjusted to have about a ¼ wave opticalthickness (index times physical thickness) for 340 nm light andprogressing to the thickest layers which can be adjusted to be about ¼wave thick optical thickness for 420 nm light.

The UV-reflective optical layer stack comprises first optical layers andsecond optical layers, which have an average individual layer thicknessof about one quarter the wavelength of UV-light, and a layer pairthickness of about one half the wavelength of UV-light. For example, at400 nanometers (nm) the average individual layer thickness would beabout 100 nm, and the average layer pair thickness would be about 200nm. Similarly, at 350 nm the average individual layer thickness wouldabout 87 nm, and the average layer pair thickness would be about 175 nm.Of course, true thickness will typically vary such that a range ofthickness (and hence reflective response) will be obtained. Further, thethickness of individual first and/or second optical layers may bevaried, for example, according to a thickness gradient in which thefirst and/or second layers gradually become thicker (or thinner) withrespect to the first major surface, thereby achieving a broadbandreflective response (i.e., UV-light and VIS/IR-light).

Since reflectivity at a given interface is typically relatively low, thenumber of the optical layers is typically large (e.g., at least 100,250, 500, or even at least 1000 optical layers) such that the totallight reflectivity is high.

In order for an optical layer stack to function efficiently, the firstand second optical layers (or, as described hereinbelow, third andfourth optical layers in the case of a UV/IR optical layer stack) atleast a portion, typically all or substantially all, of any two opticallayers that are in intimate contact (i.e., a layer pair) have arefractive index difference that results in reflection of a portion ofany light traversing the interface between the optical layers.

An exemplary layer pair 164 a is shown in FIG. 1B. Typically, theoptical layers of a given layer pair are selected such as to besubstantially transparent to those light wavelengths at whichreflectivity is desired. Light that is not reflected at a layer pairinterface passes to the next layer pair interface where a portion of thelight is reflected and unreflected light continues on, and so on. Inthis way, an optical layer stack (e.g., a UV-reflective optical layerstack or a VIS/IR-reflective optical layer stack) with many opticallayers (e.g., more than 100 optical layers) is capable of generating ahigh degree of reflectivity, which may be broadband. The difference inrefractive index, with respect to at least one dimension, between layersof the layer pair is typically 0.05 or larger, more typically at least0.1, although lesser differences may also be used if desired.

Birefringence (e.g., caused by stretching) of optical layers is aneffective method for increasing the difference in refractive index ofthe optical layers in a layer pair. Multilayer reflective optical filmsthat include layer pairs are oriented in two mutually perpendicularin-plane axes are capable of reflecting an extraordinarily highpercentage of incident light depending on the number of optical layers,f-ratio, indices of refraction, etc., and are highly efficientreflectors. Reflectors can also be made using a combination ofuniaxially-oriented layers with in-plane indices of refraction thatdiffer significantly.

As shown in FIG. 1B, the layer pairs may be arranged as alternatinglayer pairs (e.g., ABABAB.). In other embodiments, they may also bearranged with intermediate layers such as, for example, a third opticallayer (e.g., ABCABC . . . ) or in non-alternating fashion (e.g.,ABABABCAB . . . , ABABCABDAB . . . or ABABBAABAB . . . , etc.).Typically, the layer pairs are arranged as alternating layer pairs.

The optional abrasion resistant layer may comprise any abrasionresistant material that is transparent to the wavelengths of UV-lightand VIS/IR-light reflected by the broadband reflector. Examples ofscratch resistant coatings include: a thermoplastic urethane availableas TECOFLEX from Lubrizol Advanced Materials, Inc, Cleveland, Ohio,??containing 5 weight percent of a UV absorber available as TINUVIN 405from Ciba Specialty Chemicals Corp., 2 weight percent of a hinderedamine light stabilizer available as TINUVIN 123, and 3 weight percent ofa UV-absorber available as TINUVIN 1577 from Ciba Specialty ChemicalsCorp. and a scratch resistant coating preparable by curing a curablesiliceous polymer composition available as PERMA-NEW 6000 (or PERMA-NEW6000B) CLEAR HARD COATING SOLUTION from California Hardcoating Co. ofChula Vista, Calif.

The abrasion resistant layer may have any suitable thickness, typicallydepending on the choice of abrasion resistant material used. Typicalabrasion resistant layer thicknesses are on the order of 1 to 10micrometers, more typically 3 to 6 micrometers, although otherthicknesses may be used.

The optional abrasion resistant layer may optionally include at leastone antisoiling component. Examples of antisoiling components includefluoropolymers, silicone polymers, and titanium dioxide particles,fluoropolymers, silicone polymers, titanium dioxide particles,polyhedral oligomeric silsesquioxanes (e.g., as available as POSS fromHybrid Plastics of Hattiesburg, Miss.), and combinations thereof. Theabrasion resistant layer may also comprise a conductive filler,typically a transparent conductive filler. The abrasion resistant layermay also comprise an inherently static dissipative polymer such as thoseavailable as STATRITE X5091 or STATRITE M809 from Lubrizol Corp.

If present, the optional tie layer facilitates adhesion of theVIS/IR-reflective metal layer to the UV-reflective multilayer opticalfilm. The tie layer may be organic (e.g., a polymeric layer oradhesive), inorganic, or other. Exemplary inorganic tie layers includeamorphous silica, silicon monoxide, and metal oxides (e.g., tantalumpentoxide, titanium dioxide, and aluminum oxide). The tie layer may beprovided by any suitable means, including vapor coating, solventcasting, and powder coating techniques. In order that it does notdegrade performance of the broadband reflector, the optional tie layeris typically substantially not absorptive of light (e.g., having anabsorbance of less than 0.1, less than 0.01, less than 0.001, or lessthan 0.0001) over the wavelength range of from greater than 400 to 2494nm.

The VIS/IR-reflective metal layer may comprise any metal or metals thatreflects VIS/IR light, typically efficiently. In general, theVIS/IR-reflective metal layer should be at least about 100 nm thick toensure high reflectivity. Exemplary metals include silver, copper,aluminum, copper on silver, nichrome, stainless steel, and nickel. Themetal layer may be secured to the UV-reflective multilayer optical filmby any suitable technique including lamination, sputtering, and vaporcoating. For example, the metal layer may be provided by laminating theUV-reflective multilayer optical film to polished aluminum stock, silvercoated aluminum stock, polished stainless steel stock, and silver coatedglass (including retro-fitting existing silver coated glass CSPtroughs). In some embodiments, the metal layer includes aVIS/IR-reflective foil, a vapor coated metal surface (e.g., frontsurface VIS/IR-reflective metal layers on a glass or polymeric backing),or metallic sheet stock.

Referring now to FIG. 2A, a broadband reflector 200 according to anotherexemplary embodiment of the present disclosure has UV-reflectivemultilayer optical film 110 with first major surface 115, UV-reflectiveoptical layer stack 140, optional tie layer 120, and optional abrasiveresistant hardcoat 150. VIS/IR-reflective multilayer optical film 280comprises VIS/IR-reflective optical layer stack 240. Optional adhesivelayer 225 is disposed on at least a portion of VIS/IR-reflectivemultilayer optical film 280 opposite UV-reflective multilayer opticalfilm 110.

Referring now to FIG. 2B, VIS/IR-reflective optical layer stack 240comprises third optical layers 260 a, 260 b, . . . , 260 n (collectivelythird optical layers 260) and fourth optical layers 262 a, 262 b, . . ., 262 n (collectively fourth optical layers 262) analogous to the firstand second optical layers 160, 162, but of different thickness andoptionally composition. VIS/IR-reflective optical layer stack 240 may beconstructed generally as in UV-reflective optical layer stack 140 exceptfor optical layer thicknesses that will differ due to the wavelengthdifference. Materials selection for third and fourth optical layers maybe the same or different as the first and second optical layers ofUV-reflective optical layer stack 140 as desired.

Referring again to FIG. 2A, UV-absorbing layer 290 is disposed (e.g.,sandwiched) between UV-reflective multilayer optical film 110 andVIS/IR-reflective multilayer optical film 280, which is reflective toVIS/IR-light.

The UV-absorbing layer is disposed between the UV-reflective multilayeroptical film and the VIS/IR multilayer optical film, and serves toabsorb UV-light that would otherwise impinge on the VIS/IR-reflectivemultilayer optical film and could lead to degradation over time. Solarlight, in particular the ultraviolet radiation from 280 to 400 nm, caninduce degradation of plastics, which in turn results in color changeand deterioration of optical and mechanical properties. Inhibition ofphoto-oxidative degradation is important for outdoor applicationswherein long term durability is mandatory. The absorption of UV-light bypolyethylene terephthalates, for example, starts at around 360 nm,increases markedly below 320 nm, and is very pronounced at below 300 nm.Polyethylene naphthalates strongly absorb UV-light in the 310-370 nmrange, with an absorption tail extending to about 410 nm, and withabsorption maxima occurring at 352 nm and 337 nm. Chain cleavage occursin the presence of oxygen, and the predominant photooxidation productsare carbon monoxide, carbon dioxide, and carboxylic acids. Besides thedirect photolysis of the ester groups, consideration has to be given tooxidation reactions, which likewise form carbon dioxide via peroxideradicals.

The UV-absorbing layer comprises a polymer and a UV-absorber. Typically,the polymer is a thermoplastic polymer, but this is not a requirement.Examples of suitable polymers include polyesters (e.g., polyethyleneterephthalate), fluoropolymers, acrylics (e.g., polymethylmethacrylate), silicone polymers (e.g., thermoplastic siliconepolymers), styrenic polymers, polyolefins, olefinic copolymers (e.g.,copolymers of ethylene and norbornene available as TOPAS COC from TopasAdvanced Polymers of Florence , Ky.), silicone copolymers,fluoropolymers, and combinations thereof (e.g., a blend of polymethylmethacrylate and polyvinylidene fluoride).

The UV-absorbing layer protects the VIS/IR-reflective optical layerstack from UV-light caused damage by absorbing any UV-light that maypass through the UV-reflective optical layer stack. In general, theUV-absorbing layer may include any polymeric composition (i.e., polymerplus additives), including pressure-sensitive adhesive compositions,that is capable of withstanding UV-light for an extended period of time.

A variety of ultraviolet light absorbing and stabilizing additives aretypically incorporated into the UV-absorbing layer to assist in itsfunction of protecting the VIS/IR-reflective multilayer optical film.Non-limiting examples of the additives include one or more compoundsselected from ultraviolet light absorbers, hindered amine lightstabilizers, antioxidants, and combinations thereof

UV stabilizers such as UV-absorbers are chemical compounds that canintervene in the physical and chemical processes of photoinduceddegradation. The photooxidation of polymers from ultraviolet radiationcan therefore be prevented by use of a UV-absorbing layer that containsat least one UV-absorber to effectively absorb light at wavelengths lessthan about 400 nm. UV-absorbers are typically included in theUV-absorbing layer in an amount that absorb at least 70 percent,typically 80 percent, more typically greater than 90 percent, or evengreater than 99 percent of incident light in a wavelength region from180 to 400 nm.

Typical UV-absorbing layer thicknesses are from 10 to 500 micrometers,although thinner and thicker UV-absorbing layers may also be used.Typically, the UV-absorber is present in the UV-absorbing layer in anamount of from 2 to 20 percent by weight, but lesser and greater levelsmay also be used.

One exemplary UV-absorber is a benzotriazole compound,5-trifluoromethyl-2-(2-hydroxy-3-alpha-cumyl-5-tert-octylphenyl)-2H-benzotriazole.Other exemplary benzotriazoles include2-(2-hydroxy-3,5-di-alpha-cumylphehyl)-2H-benzotriazole,5-chloro-2-(2-hydroxy-3-tert-butyl-5-methylphenyl)-2H-benzotiazole,5-chloro-2-(2-hydroxy-3,5-di-tert-butylphenyl)-2H-benzotriazole,2-(2-hydroxy-3,5-di-tert-amylphenyl)-2H-benzotriazole,2-(2-hydroxy-3-alpha-cumyl-5-tert-octylphenyl)-2H-benzotriazole,2-(3-tert-butyl-2-hydroxy-5-methylphenyl)-5-chloro-2H-benzotriazole.Additional exemplary UV-absorbers include2(-4,6-diphenyl-1-3,5-triazin-2-yl)-5-hexcyloxy-phenol, and thoseavailable from Ciba Specialty Chemicals Corp. as TINUVIN 1577 andTINUVIN 900. In addition, UV-absorber(s) can be used in combination withhindered amine light stabilizer(s) (HALS) and/or antioxidants. ExemplaryHALSs include those available from Ciba Specialty Chemicals Corp. asCHIMASSORB 944 and TINUVIN 123. Exemplary antioxidants include thoseavailable as IRGANOX 1010 and ULTRANOX 626 from Ciba Specialty ChemicalsCorp.

In addition to adding UVA, HALS, and antioxidants to the UV-absorbinglayer, the UVA, HALS, and antioxidants can be added to other layerspresent in broadband reflectors according to the present disclosure.

Other additives may be included in the UV-absorbing layer. Smallparticle non-pigmentary zinc oxide and titanium oxide can also be usedas blocking or scattering additives in the UV-absorbing layer. Forexample, nanoscale (i.e., nanometer-scale) particles can be dispersed inpolymer or coating substrates to minimize ultraviolet radiationdegradation. The nanoscale particles are transparent to visible lightwhile either scattering or absorbing harmful UV radiation therebyreducing damage to thermoplastics. U.S. Pat. No. 5,504,134 (Palmer etal.) describes attenuation of polymer substrate degradation due toultraviolet radiation through the use of metal oxide particles in a sizerange of about 0.001 micrometer to about 0.20 micrometer in diameter,and more preferably from about 0.01 to about 0.15 micrometers indiameter. U.S. Pat. No. 5,876,688 (Laundon) teaches a method forproducing micronized zinc oxide that are small enough to be transparentwhen incorporated as ultraviolet light (UV) blocking and/or scatteringagents in paints, coatings, finishes, plastic articles, and cosmetics,which are well suited for use in the present invention. These fineparticles such as zinc oxide and titanium oxide with particle sizeranged from 10-100 nm that can attenuate UV radiation are commerciallyavailable from Kobo Products, Inc., South Plainfield, N.J. Flameretardants may also be incorporated as an additive in the UV-absorbinglayer.

The thickness of the UV-absorbing layer is dependent upon an opticaldensity target at specific wavelengths as calculated by the Beer-LambertLaw. In typical embodiments, the UV-absorbing layer has an opticaldensity greater than 3.5 at 380 nm; greater than 1.7 at 390; and greaterthan 0.5 at 400 nm. Those of ordinary skill in the art will recognizethat the optical densities must remain fairly constant over the extendedlife of the article in order to provide the intended protectivefunction.

In general, UV-reflective multilayer optical films and VIS/IR-reflectivemultilayer optical films are specular reflectors, although this is not arequirement. Both multilayer reflective optical films rely on index ofrefraction differences between at least two different materials(typically polymeric materials (e.g., polymers or combinations ofpolymers)).

The first and third optical layers are typically birefringent layersthat comprise respective first and third polymers, which may be singlepolymers or combinations of polymers, which are uniaxially-oriented orbiaxially-oriented to create a sufficiently high refractive indexdifference between the first and second optical layers or third andfourth optical layers, although this is not a requirement. Highreflectivity can also be achieved by increasing the number of layers foroptical pairs with less refractive difference between first and secondoptical layers. In those embodiments wherein the first optical layer isbirefringent, the first polymer is typically capable of developing alarge birefringence when stretched. Depending on the application, thebirefringence may be developed between two orthogonal directions in theplane of the film, between one or more in-plane directions and thedirection perpendicular to the film plane, or a combination of these.The first polymer should maintain birefringence after stretching, sothat the desired optical properties are imparted to the finished film.

The second and fourth optical layers can be layers comprising respectivesecond and fourth polymers, which may be single polymers or combinationsof polymers, that are birefringent and uniaxially- or biaxially-orientedor the second optical layers can have an isotropic index of refractionthat is different from at least one of the indices of refraction of therespective first and third optical layers after orientation. The secondand fourth polymers advantageously develop little or no birefringencewhen stretched, or develops birefringence of the opposite sense(positive-negative or negative-positive), such that its film-planerefractive indices differ as much as possible from those of the first(or third) polymers in the finished film. For most applications, it isadvantageous that neither the first polymer nor the second polymerabsorbance incident light within the bandwidth of interest for the filmin question, resulting in all incident light within the bandwidth beingeither reflected or transmitted. However, some level of absorbance maybe acceptable for some applications.

The first, second, third, and fourth optical layers may comprise one ormore polymers (including miscible polymer blends). Useful classes ofpolymers include polyesters and polycarbonates.

Polyesters may be derived, for example, from ring-opening additionpolymerization of a lactone, or by condensation of a dicarboxylic acid(or derivative thereof such as, for example, a diacid halide or adiester) with a diol. Exemplary dicarboxylic acids include:2,6-naphthalenedicarboxylic acid; terephthalic acid; isophthalic acid;phthalic acid; azelaic acid; adipic acid; sebacic acid;norbornenedicarboxylic acid; bicyclooctanedicarboxylic acid;1,6-cyclohexanedicarboxylic acid; t-butyl isophthalic acid, trimelliticacid, sodium sulfonated isophthalic acid; 4,4′-biphenyldicarboxylicacid. Acid halides and lower alkyl esters of these acids, such as methylor ethyl esters may also be used as functional equivalents. The term“lower alkyl” refers, in this context, to alkyl groups having from oneto ten carbon atoms. Exemplary diols include: ethylene glycol; propyleneglycol; 1,4-butanediol; 1,6-hexanediol; neopentyl glycol; polyethyleneglycol; diethylene glycol; tricyclodecanediol;1,4-cyclohexanedimethanol; norbornanediol; bicyclooctanediol;trimethylolpropane; pentaerythritol; 1,4-benzenedimethanol; bisphenol A;1,8-dihydroxybiphenyl; and 1,3-bis (2-hydroxyethoxy)benzene.

Exemplary polymers useful for forming birefringent optical layersinclude, for example: polyethylene terephthalates (PETs) (e.g., a PEThaving an inherent viscosity of 0.74 dL/g, available from EastmanChemical Company (Kingsport, Tenn.)); polyethylene 2,6-naphthalates(PENs); copolyesters derived from naphthalenedicarboxylic acid, anadditional dicarboxylic acid, and a diol (coPENs) (e.g., a polyesterderived through co-condensation of 90 equivalents of dimethylnaphthalenedicarboxylate, 10 equivalents of dimethyl terephthalate, and100 equivalents of ethylene glycol, and having an intrinsic viscosity(IV) of 0.48 dL/g, and an index of refraction is approximately 1.63);polyether imides; and polyester/non-polyester combinations; polybutylene2,6-naphthalates (PBNs); modified polyolefin elastomers, e.g., asavailable as ADMER (e.g., ADMER SE810) thermoplastic elastomers fromMitsui Chemicals America, Inc. of Rye Brook, N.Y.; and copolyestersderived from terephthalic acid such as those described in U.S. Pat. No.6,449,093 B2 (Hebrink et al.) or U.S. Pat. App. Publ. No. 2006/0084780A1 (Hebrink et al.); thermoplastic polyurethanes (TPUs) (e.g., asavailable as ELASTOLLAN TPUs from BASF Corp. of Florham Park, N.J. andas TECOFLEX or STATRITE TPUs (e.g., STATRITE X5091 or STATRITE M809)from The Lubrizol Corp. of Wickliffe, Ohio); and syndiotacticpolystyrenes (sPSs), which is particularly useful due to its lowUV-light absorbance; and combinations thereof.

PEN has a large positive stress optical coefficient, retainsbirefringence effectively after stretching, and has little or noabsorbance within the visible range. PEN also has a large index ofrefraction in the isotropic state. Its refractive index for polarizedincident light of 550 nm wavelength increases when the plane ofpolarization is parallel to the stretch direction from about 1.64 to ashigh as about 1.9. Increasing molecular orientation increases thebirefringence of PEN. The molecular orientation may be increased bystretching the material to greater stretch ratios and holding otherstretching conditions fixed. coPENs such as those described in U.S. Pat.Nos. 6,352,761 B1 and 6,449,093 B2 (both to Hebrink et al.) areparticularly useful for their low temperature melt processingcapability.

Exemplary melt-processible polymers useful in non-birefringent opticallayers include: polyesters (e.g., polycyclohexanedimethyleneterephthalate commercially available from Eastman Chemical Co,Kingsport, Tenn.); polysulfones; polyurethanes; polyamides; polyimides;polycarbonates; polydimethylsiloxanes; polydiorganosiloxane polyoxamideblock copolymers (OTPs) such as, e.g., those described in U.S. Publ.Pat. Appln. Nos. 2007/0148474 A1 (Leir et al.) and 2007/0177272 A1(Benson et al.); fluoropolymers including, e.g., homopolymers such aspolyvinylidene difluoride (PVDFs), copolymers such as copolymers oftetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride(THVs), copolymers of hexafluoropropylene, tetrafluoroethylene, andethylene (HTEs); copolymers of tetrafluoroethylene and norbornene;copolymers of ethylene and tetrafluoroethylene (ETFEs); copolymers ofethylene and vinyl acetate (EVAs); copolymers of ethylene andchlorotrifluoroethylene (ECTFEs), and fluoroelastomers; acrylics such aspolymethyl methacrylate (PMMA, e.g., as available as CP71 and CP80 fromIneos Acrylics, Inc. of Wilmington, Del.) and copolymers of methylmethacrylate (coPMMAs) such as, e.g., a coPMMA made from 75 weightpercent methyl methacrylate and 25 weight percent ethyl acrylate(available from Ineos Acrylics, Inc., as PERSPEX CP63) and a coPMMAformed from methyl methacrylate and n-butyl methacrylate; styrenicpolymers; vinyl acetate copolymers (e.g., ethylene vinyl acetatecopolymers); copolymers of ethylene and a cyclic olefin (COCs); blend ofPMMA and PVDF (e.g., as available from Solvay Polymers, Inc., Houston,Tex., as SOLEF); polyolefin copolymers such as poly (ethylene-co-octene)(PE-POs) available from Dow Chemical Co., Midland, Mich. as ENGAGE 8200,poly (propylene-co-ethylene) (PPPE) available from Fina Oil and ChemicalCo., Dallas, Tex. as Z9470, and a copolymer of atactic polypropylene(aPPs) and isotactic polypropylene (iPPs) available from HuntsmanChemical Corp., Salt Lake City, Utah as REXFLEX W111; and combinationsthereof. Second optical layers can also be made from a functionalizedpolyolefin, such as linear low density polyethylene-g-maleic anhydride(LLDPE-g-MA) such as that available from E. I. du Pont de Nemours & Co.,Inc., Wilmington, Del. as BYNEL 4105; and combinations thereof.

In some embodiments, the melt-processible copolymers oftetrafluoroethylene and other monomer(s), described above, includeadditional monomers such as, e.g., propylene, ethylene, norbornene,and/or perfluorinated vinyl ethers represented by the formula:

CF₂═CF(OCF₂CF(R_(f)))_(a)OR′_(f)

wherein R_(f) is a perfluoroalkyl group having from 1 to 8 (typically 1to 3 carbon atoms), and R′_(f) is a perfluoroaliphatic group, typicallya perfluoroalkyl or perfluoroalkoxy group having from 1 to 8 (typically1 to 3 carbon atoms), and a is 0, 1, 2, or 3. Exemplary perfluorinatedvinyl ethers having this formula include: CF₂═CFOCF₃,CF₂═CFOCF₂CF₂CF₂OCF₃, CF₂═CFOCF₂CF₂CF₃, CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CF₃, andCF₂═CFOCF₂CF(CF₃)OCF₂CF(CF₃)OCF₂CF₂CF₃. Exemplary melt-processiblecopolymers of tetrafluoroethylene and other monomer(s) discussed aboveinclude those available as DYNEON THV 220, DYNEON THV 230, DYNEON THV500, DYNEON THV 500G, DYNEON THV 510, DYNEON THV 510D, DYNEON THV 610,DYNEON THV 815, and DYNEON THVP 2030G from Dyneon LLC of Oakdale, Minn.Additional useful polymers are disclosed in U.S. Pat. No. 6,498,683 B2(Condo et al.) and U.S. Pat. No. 6,352,761 B1 (Hebrink et al.).

Polymers used for forming optical layers of the UV-reflective opticallayer stack typically have little or no absorbance at wavelengths above350 nm, and desirably above 300 nm.

In general, at least the second and fourth polymers should be chosen sothat in the respective optical layer stack, the refractive indexes ofthe second and fourth optical layers, in at least one direction, differsignificantly from the index of refraction of the respective first andthird optical layers in the same direction. Because polymeric materialsare typically dispersive, that is, the refractive indices vary withwavelength, these conditions must be considered in terms of a particularspectral bandwidth of interest. It will be understood from the foregoingdiscussion that the choice of a second polymer is dependent not only onthe intended application of the optical layer stack in question, butalso on the choice made for the first polymer, as well as processingconditions.

The UV-reflective and/or VIS/IR multilayer optical films may optionallyinclude one or more non-optical layers such as, for example, one or moreskin layers or one or more interior non-optical layers, such as, forexample, protective boundary layers between packets of optical layers.Non-optical layers can be used to give the multilayer optical filmstructure or to protect it from harm or damage during or afterprocessing. For some applications, it may be desirable to includesacrificial protective skins, wherein the interfacial adhesion betweenthe skin layer(s) and the optical layer stack is controlled so that theskin layers can be stripped from the optical layer stack before use.

Materials may be chosen for the non-optical layers that impart orimprove properties such as, for example, tear resistance, punctureresistance, toughness, weatherability, and solvent resistance of themultilayer optical body. Typically, one or more of the non-opticallayers are placed so that at least a portion of the light to betransmitted or reflected by optical layers also travels through theselayers (i.e., these layers are placed in the path of light that travelsthrough or is reflected by the first and second optical layers). Thenon-optical layers typically do not substantially affect the reflectiveproperties of the broadband reflectors over the wavelength region ofinterest. Properties of the non-optical layers such as thermal expansionand shrinkage characteristics need to be considered along with theproperties of the optical layers to give the film of the presentinvention that does not crack or wrinkle when laminated to severelycurved substrates.

The non-optical layers may be of any appropriate material and can be thesame as one of the materials used in the optical layer stack. Of course,it is important that the material chosen not have optical propertiesdeleterious to those of the optical layer stack(s). For example, thenon-optical layers should typically have little or no absorbance thatwould interfere with optical layer stack. The non-optical layers may beformed from a variety of polymers, such as polyesters, including any ofthe polymers used in the first and second optical layers. In someembodiments, the material selected for the non-optical layers is similarto or the same as the material selected for the second optical layers.The use of coPEN, coPET, or other copolymer material for skin layersreduces the breaking apart of multilayer optical film(s) due tostrain-induced crystallinity and alignment of a majority of the polymermolecules in the direction of orientation.

Typically, the polymers of the first and second optical layers (andlikewise the third and fourth optical layers), and the optionalnon-optical layers are chosen to have similar rheological properties(e.g., melt viscosities) so that they can be co-extruded without flowdisturbances. Typically, the second and fourth optical layers, skinlayers, and optional other non-optical layers have a glass transitiontemperature (T_(g)), that is either less than or equal to about 40° C.above the glass transition temperature of the respective first and thirdoptical layers.

The optional non-optical layers can be thicker than, thinner than, orthe same thickness as the various optical layers. The thickness of theoptional non-optical layers is generally at least four times, typicallyat least 10 times, and can be at least 100 times, the thickness of atleast one of the individual optical layers. The thickness of thenon-optical layers can be varied to make a multilayer reflective filmhaving a particular thickness. Optional non-optical layers may includeone or more UV-absorbers and/or stabilizers. For example, they maycontain extrusion compounded UVA absorbers such as those available asTINUVIN 1557 from Ciba Specialty Chemicals Corp., and extrusioncompounded hindered amine light stabilizers (HALS) such as thoseavailable as CHIMASSORB 944 from Ciba Specialty Chemicals Corp.

Additional coatings may also be considered non-optical layers. Otherlayers include, for example, antistatic coatings or films; flameretardants; optical coatings; anti-fogging materials, etc.

The broadband reflectors can be fabricated by methods well-known tothose of skill in the art by techniques such as for example, heatlamination and adhesive bonding. Exemplary useful adhesives (e.g.,adhesive layer 120) that may be used in fabrication of broadbandreflectors according to the present disclosure may be opticallytransparent and reasonably UV-light stable or not, and include:optically clear acrylic pressure sensitive adhesives (25 μm thickness)available from 3M Company as OPTICALLY CLEAR LAMINATING ADHESIVE 8141 oras OPTICALLY CLEAR LAMINATING ADHESIVE 8171; tackified OTP adhesives asdescribed in U.S. Pat. No. 7,371,464 B2 (Sherman et al.); andnon-silicone pressure-sensitive adhesives as described in PCT Pat.Appln. No. PCT/US08/86596 entitled “Urea-Based Pressure SensitiveAdhesives”, filed Dec. 12, 2008. In those applications wherein theadhesive is used as optional adhesive layer 125 or optional adhesivelayer 225, then optical transparency and/or stability is typically notof significance as little if any light will reach the adhesive intypical constructions. Accordingly, optional adhesive layer 125 or 225may comprise any adhesive composition such as e.g., an epoxy, urethane,silicone, or acrylic adhesive, or a combination thereof. The adhesivecomposition may comprise, for example, a pressure-sensitive adhesive, athermosetting adhesive, a hot melt adhesive, or a combination thereof.Advantageously, broadband reflectors 100 and 200 may be adhesive bondedto existing solar reflector(s). This process is useful for retrofittingexisting CSP installations.

Further considerations relating to the selection of materials, mode ofoperation, and manufacturing of multilayer optical films can be obtainedwith reference to U.S. Pat. No. 5,882,774 (Jonza et al.); U.S. Pat. No.6,157,490 (Wheatley et al.); U.S. Pat. No. 6,207,260 B1 (Wheatley etal.); 6,783,349 B2 (Neavin et al.); U.S. Pat. No. 6,827,886 B2 (Neavinet al.); U.S. Pat. No. 6,179,948 B1 (Merrill et al.); 6,808,658 B2(Stover); and U.S. Pat. No. 6,830,713 B2 (Hebrink et al.).

Broadband reflectors according the present disclosure may be planar orcurved (e.g., parabolic or parabolic trough-shaped) and are typically inthe form of a compliant sheet or film, although this is not arequirement. For example, they may be combined with a rigid member(e.g., a backing support) that imparts and retains a particular shape tothe broadband reflector surface (e.g., a curved reflective surface). Forpurposes of the present disclosure, the term “compliant” is anindication that the broadband reflector is dimensionally stable yetpossesses a pliable characteristic that enables subsequent molding orshaping into various forms. In some exemplary embodiments, broadbandreflectors according to the present disclosure may be thermoformed intovarious shapes or structures for specific end use applications.

Broadband reflectors according to the present disclosure may beconverted, for example, by thermoforming, vacuum forming, shaping,rolling, or pressure forming, into shapes and/or dimensions (e.g.,curved surfaces and parabolic surfaces) conventionally used forreflectors such as, for example, solar concentrators used inconcentrated solar power systems. Additionally, the broadband reflectorsmay be reinforced, for example, by injection cladding, corrugation, oraddition of ribs, foam spacer layers, or honeycomb structures to improveits dimensional stability. One exemplary reinforcing material is twinwall polycarbonate sheeting, e.g., as available as SUNLITE MULTIWALLPOLYCARBONATE SHEET from Palram Americas, Inc. of Kutztown, Pa.Thermoforming of multilayer optical films is generally described in U.S.Pat. No. 6,788,463 B2 (Merrill et al.).

Advantageously, broadband reflectors according to the presentdisclosure, especially if formed into solar concentrators, are capableof concentrating high levels of sunlight at a fraction of the weight ofconventional solar concentrators.

An exemplary concentrated solar power system 300 is depictedschematically in FIG. 3. Concentrated solar power system 300 comprisesbroadband reflectors 310 connected to celestial tracking mechanism 320that is capable of aligning direct solar radiation from broadbandreflectors 310 onto hollow receiver 330. A heat transfer fluid 340circulates by means of pump 360 through the hollow receiver 330 where itis heated by concentrated solar radiation. The heated heat transferfluid 340 is then directed to an electrical generator 350 (e.g., a steamturbine) when the thermal energy is converted to electrical energy. Inanother embodiment, the heat transfer fluid may be directed to a heatexchanger instead of the electrical generator, where the heat content istransferred to a liquid medium such as, for example, water that isconverted to steam which drives the electrical generator.

Another exemplary concentrated solar power system 400 is depictedschematically in FIG. 4. Concentrated solar power system 400 comprisesparabolic trough-shaped broadband reflectors 410 connected to celestialtracking mechanism 420 that is capable of aligning direct solarradiation from broadband reflectors 410 onto hollow receiver 430. A heattransfer fluid 440 circulates by means of pump 460 through the hollowreceiver 430 where it is heated by concentrated solar radiation. Theheated heat transfer fluid 440 is then directed to a thermal heatingsystem 450 where the thermal energy is converted to electrical energy.

The hollow receivers may be transparent or opaque and should typicallybe made of material (e.g., metal or glass) that is capable ofwithstanding the light and heat directed upon it by the broadbandreflectors. Exemplary heat transfer fluids include water, water/glycolmixtures, brine, molten salts, and oils, with the selected typicallybeing dictated by application requirements and cost. Often the hollowreceivers comprise an interior pipe coated with a solar absorbingmaterial disposed inside an exterior transparent (e.g., glass) pipe,although other configurations may also be used. In some embodiments, theheated heat transfer fluid flowing through the solar absorbing hollowreceiver exchanges heat with water to create steam that drives anelectric generator.

Further enhancements in the concentrated solar polar system output maybe achieved when anti-reflective surface structured films or coatingsare applied to the front surface of the hollow receiver. Surfacestructures in the films or coating typically change the angle ofincidence of light such that it enters the polymer and hollow receiverbeyond the critical angle and is internally reflected, leading to moreabsorption by the hollow receiver. Such surface structures can be in theshape, for example, of linear prisms, pyramids, cones, or columnarstructures. For prisms, typically the apex angle of the prisms is lessthan 90 degrees (e.g., less than 60 degrees). The refractive index ofthe surface structured film or coating is typically less than 1.55(e.g., less than 1.50). These anti-reflective surface structured filmsor coatings can be made durable and easily cleanable with the use ofinherently UV stable and hydrophobic or hydrophilic materials.Anti-reflective coatings (e.g., nanostructured coatings or lowrefractive index coatings) could also be applied to the interior glasssurface of the hollow receiver. Durability of the anti-reflectivecoatings or films can be enhanced with the addition of inorganicnano-particles.

Broadband reflectors according to the present disclosure may also beuseful, for example, for concentrated photovoltaic systems. For example,a broadband reflector disclosed herein may be useful when placed inproximity to a multi junction GaAs cell, which has an absorptionbandwidth from about 350 nm to about 1750 nm, or a mono-crystallinesilicon photovoltaic cell having an absorption bandwidth of about 400 nmto about 1150 nm. In some embodiments, a thermal management device(e.g., in the form of ribs, pins, or fins) may be used to dissipate heatfrom the solar cell.

FIG. 7 shows a tracking device 700, which may be useful, for example, inthe concentrated solar power system 400 shown in FIG. 4. Tracking device700 comprises parabolic trough-shaped broadband reflectors 710 withhollow receiver 730 placed at the focal line. Two rods 770 extendingoutside the end pieces 712 of a trough-shaped broadband reflector 710are used to connect the trough to a frame 720 and a crossbar 722,respectively, at each end of the assembly. The crossbar 722 can beconnected to a driving mechanism. With a plurality of trough-shapedbroadband reflectors 710 being pivotally positioned in a pair ofparallel stationary frames, as shown in FIG. 7, the crossbars 722 towhich each trough 710 is attached can, in some embodiments,simultaneously pivot all of the troughs about their axes. Thus, theorientation of all the trough-shaped broadband reflectors 710 can becollectively adjusted to follow the sun movement in unison. AlthoughFIG. 7 shows two crossbars 722, one on each side of the trough 710, itis possible to use only one crossbar. In some embodiments of trackingdevice 700 shown in FIG. 7, the trough-shaped broadband reflector 710 isaligned in the east-west direction with a rotational freedom in thenorth-south direction typically not less than 10 degrees, 15 degrees, 20degrees, or 25 degrees, for example, for adjustments to track the sunthrough seasonal variations (i.e., through the different paths betweenequinox and solstice). When the hollow receiver 730 is incorporated intoa linear parabolic trough-shaped broadband reflector 710 tilted towardthe south, the incident solar irradiance enters within the acceptanceangle of the linear parabolic reflector. The aperture of the paraboladetermines how often the position of the trough-shaped broadbandreflector 710 must be changed (e.g., hourly, daily, or less frequently).In some embodiments of tracking devices 700 shown in FIG. 7, the hollowreceiver 730 is aligned in the north-south direction, and the rotationalfreedom in the east-west direction is typically not less than 90degrees, 120 degrees, 160 degrees, or 180 degrees, for example, fortracking adjustments following the sun as it moves across the skythroughout the day. In some of these embodiments, the frame can bemounted, for example, to a back board (not shown) for the solarcollection device, which back board may comprise a mechanism foradjusting tilt to track the sun through seasonal variations (e.g., inthe north-south direction). Although trough-shaped broadband reflectors710 shown in FIG. 7, have parabolic shapes, other shapes may be used(e.g., hyperbolic, elliptical, tubular, or triangular). Additionalcelestial tracking mechanisms which allow the solar concentrating mirrorand/or the solar cell to pivot in two directions and which may be usefulfor solar collection devices disclosed herein are described in U.S. Pat.App. Pub. No. 2007/0251569 (Shan et al.).

Another embodiment of a tracking device useful for the concentratedsolar power systems disclosed herein is illustrated in FIGS. 8a, 8b, and8c . In this embodiment, array 800 comprises hollow receivers 830 andlouvers 810 comprising the broadband reflector according to any of theembodiments disclosed herein pivotally mounted adjacent the hollowreceivers. A louver can comprise, for example, the broadband reflectordisclosed herein applied onto a substrate (e.g., a glass sheet,polymeric sheet, or polymer fiber composite) or a free-standingbroadband reflector. In some embodiments, the louver comprises abroadband reflector disclosed herein laminated to a polymer sheet (e.g.,PMMA). The louver may be directly attached to either side of the hollowreceiver (e.g., with hinges) as shown in FIG. 8a, 8b , or 8 c, or thelouver may be pivotally mounted on a frame that also holds the hollowreceiver. In some embodiments, two louvers are associated with (e.g.,hinged to) each hollow receiver.

In FIGS. 8a, 8b, and 8c , the louvers 810 are oriented toward themorning, mid-day, and evening sun, respectively. The louvers 810 trackthe sun and enable increased capture of sunlight 828 by hollow receivers830. As a result, typically fewer hollow receivers 830 are needed in anarray 800. The array 800 shown in FIGS. 8a and 8c may be especiallyeffective at increasing the capture of sunlight in the mornings andevenings. The louvers can move independently with rotational freedom(e.g., in the east-west direction) typically not less than 90 degrees,120 degrees, 160 degrees, or 180 degrees, for example, for trackingadjustments following the sun as it moves across the sky throughout theday. Optionally, the array 800 can be mounted, for example, to one ormore back boards (not shown), which may comprise a mechanism foradjusting tilt to track the sun through seasonal variations (e.g., inthe north-south direction). The louvers may be planar, substantiallyplanar, or curved in shape.

Solar thermal arrays 800 with louver solar trackers 810 can be made witha lower profile and lighter weight than solar thermal trackers. In someembodiments of array 800, hollow receivers 830 having widths of 1 inch(2.54 cm) or less can be used to minimize the depth profile of thearray. Arrays could also be designed with larger hollow receivers (e.g.,widths of 6-inch (15 cm), 12-inch (30.5 cm), 21-inch (53 cm), orhigher). Thus, the arrays 800 can be designed to fit a number ofapplications including use on roof tops. Embodiments wherein the hollowreceivers 830 are stationary and the louvers 810 are pivotally mountedmay be advantageous (e.g., in design and/or cost) over tracking systemswhich require movement of the hollow receivers. Solar concentration canbe adjusted, for example, with the size of the broadband reflectorrelative to the hollow receiver and the reflectors's angle relative tothe hollow receiver to optimize the solar concentration ratio for adesired geographic location.

Movement of reflectors 710 shown in FIG. 7 or louvers 810 shown in FIGS.8a, 8b, and 8c can be controlled by a number of mechanisms (e.g., pistondriven levers, screw driven levers, pulley driven cables, and camsystems). Software can also be integrated with the tracking mechanismbased on GPS coordinates to optimize the position of the mirrors.Feedback control loops based on solar absorber temperature can be usedto maximize solar thermal energy generation.

Objects and advantages of this disclosure are further illustrated by thefollowing non-limiting examples, but the particular materials andamounts thereof recited in these examples, as well as other conditionsand details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in theExamples and the rest of the specification are by weight.

Preparation of Oxalylamidopropyl-terminated Polydimethylsiloxane

Polydimethylsiloxanediamine (830.00 grams (g);14,000 g/mole) was placedin a 2-liter, 3-neck resin flask equipped with a mechanical stirrer,heating mantle, nitrogen inlet tube (with stopcock), and an outlet tube.The flask was purged with nitrogen for 15 minutes and then, withvigorous stirring, diethyl oxalate (33.56 grams) was added dropwise.This reaction mixture was stirred for approximately one hour at roomtemperature and then for 75 minutes at 80° C. The reaction flask wasfitted with a distillation adaptor and receiver. The reaction mixturewas heated under vacuum (133 Pa) for 2 hours at 120° C., and then 30minutes at 130° C. until no further distillate was able to be collected.The reaction mixture was cooled to room temperature. Gas chromatographicanalysis of the clear, mobile liquid showed that no detectable level ofdiethyl oxalate remained. The ester equivalent weight was determinedusing ¹H NMR (equivalent weight equal to 7,916 grams/equivalent) and bytitration (equivalent weight equal to 8,272 grams/equivalent).

Preparation of OTP/Into a 20° C. 10-gallon (37.85-Liter) stainless steelreaction vessel was placed 18158.4 grams of oxalylamidopropyl-terminatedpolydimethylsiloxane (titrated molecular weight=14,890 g/mole) that wasprepared generally as in the Preparation of Oxalylamidopropyl-terminatedPolydimethylsiloxane (above), but with the volumes adjusted accordingly.The vessel was subjected to agitation (75 revolutions per minute (rpm)),and purged with nitrogen flow and vacuum for 15 minutes. The kettle wasthen heated to 80° C. over the course of 25 minutes. Ethylenediamine(73.29 grams, GFS Chemicals) was vacuum charged into the kettle,followed by 73.29 grams of toluene (also vacuum charged). The kettle wasthen pressurized to one pound per square inch (7 kPa) applied pressureand heated to a temperature of 120° C. After 30 minutes, the kettle washeated at 150° C. Once a temperature of 150° C. was reached, the kettlewas vented over the course of 5 minutes. The kettle was subjected topartial vacuum (approximately 65 mm Hg, 8.67 kPa) for 40 minutes toremove the ethanol and toluene. The kettle was then pressured to 2pounds per square inch (14 kPa) applied pressure and the resultantviscous molten polymer was then drained into polytetrafluoroethylenecoated trays and allowed to cool. The cooled silicone polyoxamideproduct, polydiorganosiloxane polyoxamide block copolymer OTP1, was thenground into fine pellets.

Example 1

A UV-reflective multilayer optical film was made with first opticallayers created from polyethylene terephthalate available as EASTAPAK7452 from Eastman Chemical of Kingsport, Tenn. (PET1) and second opticallayers created from a copolymer of 75 weight percent methyl methacrylateand 25 weight percent ethyl acrylate (available from Ineos Acrylics,Inc. of Memphis, Tenn. as PERSPEX CP63) (coPMMA1). The PET1 and CoPMMA1were coextruded through a multilayer polymer melt manifold to form astack of 223 optical layers. The layer thickness profile (layerthickness values) of this UV reflector was adjusted to be approximatelya linear profile with the first (thinnest) optical layers adjusted tohave about a ¼ wave optical thickness (index times physical thickness)for 340 nanometers (nm) light and progressing to the thickest layerswhich were adjusted to be about ¼ wave thick optical thickness for 420nm light. Layer thickness profiles of such films can be adjusted toprovide for improved spectral characteristics using the axial rodapparatus taught in U.S. Pat. No. 6,783,349 (Neavin et al.) combinedwith layer profile information obtained with microscopic techniques.

In addition to these optical layers, non-optical protective skin layersof PET1 (101 micrometers thickness each) were coextruded on either sideof the optical stack. This multilayer coextruded melt stream was castonto a chilled roll at 22 meters per minute creating a multilayer castweb approximately 1400 micrometers (55 mils) thick. The multilayer castweb was then heated in a tenter oven at 95° C. for about 10 secondsprior to being biaxially oriented to a draw ratio of 3.3×3.5. Theoriented multilayer film was further heated at 225° C. for 10 seconds toincrease crystallinity of the PET layers. The UV-reflective multilayeroptical film (FILM 1) was measured with a spectrophotometer (LAMBDA 950UV/VIS/NIR SPECTROPHOTOMETER from Perkin-Elmer, Inc. of Waltham, Mass.)to have an average reflectivity of 97.8 percent over a bandwidth of340-420 nm. A 10 nm thick layer of alumina was vapor coated on one sideof the film. A 110 nm thick layer of silver was vapor coated onto thealumina layer, and then a 20 nm thick layer of copper was vapor coatedover the silver layer for improved corrosion resistance.

Average solar reflectivity (for light impinging the surface of the filmopposite the copper layer) of the resultant film was measured and isreported in FIG. 5. Then the reflectance spectrum was weight averagedwith the Standard Air Mass 1.5 Direct Normal and Hemispherical SpectralSolar Irradiance for 37° Sun-Facing Tilted Surface (according to ASTM G173-03, updated March 2006 and entitled “Standard Tables for ReferenceSolar Spectral Irradiances: Direct Normal and Hemispherical on 37°Tilted Surface” resulting in a calculated 96.3 percent reflectivity overa bandwidth of from 300 nm to 2494 nm.

Example 2

A UV-reflective multilayer optical film was made with first opticallayers created from PET1 and second optical layers created from CoPMMA1.PET1 and CoPMMA1 were coextruded through a multilayer polymer meltmanifold to form 223 optical layers. The layer thickness profile (layerthickness values) of this UV reflector was adjusted to be approximatelya linear profile with the first (thinnest) optical layers adjusted tohave about a ¼ wave optical thickness (index times physical thickness)for 340 nm light and progressing to the thickest layers which wereadjusted to be about ¼ wave thick optical thickness for 420 nm light.Layer thickness profiles of such films can be adjusted to provide forimproved spectral characteristics using the axial rod apparatus taughtin U.S. Pat. No. 6,783,349 (Neavin et al.) combined with layer profileinformation obtained with microscopic techniques.

In addition to these optical layers, non-optical protective skin layersof PET1 with a thickness of 101 micrometers each were coextruded oneither side of the optical stack. This multilayer coextruded melt streamwas cast onto a chilled roll at 22 meters per minute creating amultilayer cast web approximately 1400 micrometers (55 mils) thick. Themultilayer cast web was then heated in a tenter oven at 95° C. for about10 seconds prior to being biaxially oriented to a draw ratio of 3.3×3.5.The oriented multilayer film was further heated at 225° C. for 10seconds to increase crystallinity of the PET1 layers. The resultantUV-reflective multilayer optical film (FILM 1) was measured with aspectrophotometer (LAMBDA 950 UV/VIS/NIR SPECTROPHOTOMETER fromPerkin-Elmer, Inc. of Waltham, Mass.) to have an average reflectivity of97.8 percent over a bandwidth of 350-420 nm.

A VIS/IR-light reflective multilayer optical film was made with firstoptical layers created from PET1 and second optical layers created fromCoPMMA1. PET1 and CoPMMA1 were coextruded through a multilayer polymermelt manifold to form 550 optical layers. In addition to these opticallayers, non-optical protective skin layers of PET1(?) were coextruded oneither side of the optical stack. This multilayer coextruded melt streamwas cast onto a chilled roll at 22 meters per minute creating amultilayer cast web approximately 1400 micrometers (55 mils) thick. Themultilayer cast web was then heated in a tenter oven at 95° C. for about10 seconds prior to being biaxially oriented to a draw ratio of 3.8×3.8.The oriented multilayer film was further heated at 225° C. for 10seconds to increase crystallinity of the PET1 layers. The resultantVis/IR-light reflective multilayer optical film (FILM 2) was measuredwith a LAMBDA 950 UV/VIS/NIR spectrophotometer to have an averagereflectivity of 97.1 percent over a bandwidth of 400-800 nm.

FILM 1 was laminated to FILM 2 with an acrylate-based optically clearpressure sensitive adhesive manufactured by 3M Company as OPTICALLYCLEAR LAMINATING ADHESIVE PSA 8141.

A 10 nm thick layer of alumina was then vacuum vapor coated onto theexposed surface of FILM 2. A 110 nm thick layer of silver was vacuumvapor coated onto the alumina layer, and then a 20 nm thick layer ofcopper was vacuum vapor coated over the silver layer for improvedcorrosion resistance.

Average solar reflectivity (for light impinging the surface of the filmopposite the copper layer) of the resultant film was measured and isreported in FIG. 6. Then the reflectance spectrum was weight averagedwith the Standard Air Mass 1.5 Direct Normal and Hemispherical SpectralSolar Irradiance for 37° Sun-Facing Tilted Surface (according to ASTM G173-03, updated March 2006 and entitled “Standard Tables for ReferenceSolar Spectral Irradiances: Direct Normal and Hemispherical on 37°Tilted Surface” resulting in a calculated 96.9 percent reflectivity overa bandwidth of from 300 nm to 2494 nm.

Example 3

A UV-reflective multilayer optical film was made with first opticallayers created from PET1 and second optical layers created from CoPMMA1.PET1 and CoPMMA1 were coextruded through a multilayer polymer meltmanifold to form 223 optical layers. The layer thickness profile (layerthickness values) of this UV reflector was adjusted to be approximatelya linear profile with the first (thinnest) optical layers adjusted tohave about a ¼ wave optical thickness (index times physical thickness)for 340 nm light and progressing to the thickest layers which wereadjusted to be about ¼ wave thick optical thickness for 420 nm light.Layer thickness profiles of such films can be adjusted to provide forimproved spectral characteristics using the axial rod apparatus taughtin U.S. Pat. No. 6,783,349 (Neavin et al.) combined with layer profileinformation obtained with microscopic techniques.

In addition to these optical layers, non-optical protective skin layersof PET1 of 101 micrometers thickness each were coextruded on either sideof the optical stack. This multilayer coextruded melt stream was castonto a chilled roll at 22 meters per minute creating a multilayer castweb approximately 1400 micrometers (55 mils) thick. The multilayer castweb was then heated in a tenter oven at 95° C. for about 10 secondsprior to being biaxially oriented to a draw ratio of 3.3×3.5. Theoriented multilayer film was further heated at 225° C. for 10 seconds toincrease crystallinity of the PET1 layers. The resultant UV-reflectivemultilayer optical film (FILM 1) was measured with a LAMBDA 950UV/VIS/NIR spectrophotometer to have an average reflectivity of 97.8percent over a bandwidth of 340-420 nm.

A VIS/IR-reflective multilayer optical film was made with first opticallayers created from polyethylene 2,6-naphthalate (PEN1) and secondoptical layers created from PMMA1 (an acrylic resin available under thetrade designation V044 Acrylic Resin from Arkema, Inc.). PEN1 and PMMA1were coextruded through a multilayer polymer melt manifold to form 275optical layers. In addition to these optical layers, non-opticalprotective skin layers of PEN1 were coextruded on either side of theoptical stack. This multilayer coextruded melt stream was cast onto achilled roll at 22 meters per minute creating a multilayer cast webapproximately 1400 micrometers (55 mils) thick. The multilayer cast webwas then heated in a tenter oven at 140° C. for about 10 seconds priorto being biaxially oriented to a draw ratio of 3.8×3.8. The orientedmultilayer film was further heated at 235° C. for 10 seconds to increasecrystallinity of the PEN1 layers. The resultant VIS/IR-reflectivemultilayer optical film (FILM 3) was measured with a LAMBDA 950UV/VIS/NIR spectrophotometer to have an average reflectivity of 98.6percent over a bandwidth of 400-900 nm.

Film 1 was laminated to Film 3 with an acrylate based optically clearpressure sensitive adhesive manufactured by 3M Company as OPTICALLYCLEAR LAMINATING PSA 8141.

A 10 nm thick layer of alumina was then vacuum vapor coated onto theexposed surface of FILM 3. A110 nm thick layer of silver was vacuumvapor coated onto the alumina layer, and then a 20 nm thick layer ofcopper was vacuum vapor coated over the silver layer for improvedcorrosion resistance.

Average solar reflectivity of this film was measured and then weightaveraged with the Standard Air Mass 1.5 Direct Normal and HemisphericalSpectral Solar Irradiance for 37° Sun-Facing Tilted Surface (accordingto ASTM G 173-03, updated March 2006 and entitled “Standard Tables forReference Solar Spectral Irradiances: Direct Normal and Hemispherical on37° Tilted Surface” to be 97.7 percent over a bandwidth of 300 nm to2494 nm.

Prophetic Example 1

A UV/Vis-reflective multilayer optical film would be made with firstoptical layers created from PET (polyethylene terephthalate) availableas EASTAPAK 7452 from Eastman Chemical and second optical layers createdfrom OTP1. PET and OTP1 would be coextruded thru a multilayer polymermelt manifold to create a multilayer melt stream having 550 alternatingfirst and second optical layers. The layer thickness profile (layerthickness values) of this UV reflector would be adjusted to beapproximately a linear profile with the first (thinnest) optical layersadjusted to have about a ¼ wave optical thickness (index times physicalthickness) for 350 nm light and progressing to the thickest layers whichwould be adjusted to be about ¼ wave thick optical thickness for 800 nmlight. Layer thickness profiles of such films can be adjusted to providefor improved spectral characteristics using the axial rod apparatustaught in U.S. Pat. No. 6,783,349 (Neavin et al.) combined with layerprofile information obtained with microscopic techniques.

In addition to the first and second optical layers, a pair ofnon-optical layers consisting of PET loaded with 2 weight percent of aUV-absorber available as TINUVIN 1577 from Ciba Specialty ChemicalsCorp. would be coextrusion coated as a protective skin layer on eitherside of the UV-reflective multilayer optical film. This multilayercoextruded melt stream would be cast onto a chilled roll at 22 metersper minute creating a multilayer cast web approximately 1400 micrometers(55 mils) thick. The multilayer cast web would be then heated in atenter oven at 95° C. for 10 seconds prior to being biaxially orientedto a draw ratio of 3.3×3.5. The oriented multilayer film would befurther heated at 225° C. for 10 seconds to increase crystallinity ofthe PET layers resulting in Film 4. A 10 nm thick layer of alumina wouldthen be vacuum vapor coated onto the exposed surface of FILM 4. A 110 nmthick layer of silver would then be vacuum vapor coated onto the aluminalayer, and then a 20 nm thick layer of copper would then be vacuum vaporcoated over the silver layer for improved corrosion resistance

An acrylic resin available under the trade designation V044 AcrylicResin from Arkema, Inc. of Philadelphia, Pa., would be extrusioncompounded with 3 weight percent of a UV-absorber available as TINUVIN1577 from Ciba Specialty Chemicals Corp., and 0.15 weight percent of ahindered amine light stabilizer available as CHIMASSORB 944 from CibaSpecialty Chemicals Corp., and an anhydride-modified ethylene vinylacetate polymer adhesive available as BYNEL E418 from E.I. du Pont deNemours and Co. of Wilmington, Del., and then coextrusion coated as atie layer onto FILM 4 opposite the metallic layer and simultaneouslydirected into a nip under a pressure of 893 kg/m (50 pounds per linealinch) against a casting tool having a reflector finish surface at atemperature of 90 ° F. (32° C.), at a casting line speed of 0.38 metersper second (75 feet per minute) to produce FILM 5. The coextrusioncoated layers would have a total thickness of 150 micrometers (6 mils)with a skin:tie layer thickness ratio of 20:1. The same materials couldalso be coextrusion coated onto the opposite surface of FILM 5.

A scratch resistant coating consisting of a thermally cured siliceouspolymer available as PERMA-NEW 6000 CLEAR HARD COATING SOLUTION fromCalifornia Hardcoating Co. of Chula Vista, Calif. would be coated on topof the acrylic resin skin layer.

Prophetic Example 2

A multilayer reflective reflector would be made with first opticallayers created from PET and second optical layers created from aterpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidenefluoride available as THV 2030 from Dyneon, LLC, Oakdale, Minn. PET andTHV 2030 copolymer would be coextruded thru a multilayer polymer meltmanifold to create a multilayer melt stream having 550 alternating firstand second optical layers. The layer thickness profile (layer thicknessvalues) of this UV-Vis reflector would be adjusted to be approximately alinear profile with the first (thinnest) optical layers adjusted to haveabout a ¼ wave optical thickness (index times physical thickness) for340 nm light and progressing to the thickest layers which would beadjusted to be about ¼ wave thick optical thickness for 1100 nm light.Layer thickness profiles of such films can be adjusted to provide forimproved spectral characteristics using the axial rod apparatus taughtin U.S. Pat. No. 6,783,349 (Neavin et al.) combined with layer profileinformation obtained with microscopic techniques.

In addition to the first and second optical layers, a pair ofnon-optical layers also comprised of PET loaded with 2 weight percent ofUV-absorber available as TINUVIN 1577 from Ciba Specialty ChemicalsCorp. would be coextruded as protective skin layers on either side ofthe optical layer stack. This multilayer coextruded melt stream would becast onto a chilled roll at 22 meters per minute creating a multilayercast web approximately 1400 micrometers (55 mils) in thickness. Themultilayer cast web would then be heated in a tenter oven at 95° C. for10 seconds prior to being biaxially oriented to a draw ratio of 3.3×3.5.The oriented multilayer film would then be further heated to 225° C. for10 seconds to increase crystallinity of the PET layers resulting in FILM6. A 10 nm thick layer of titania would then vacuum vapor coated ontothe exposed surface of FILM 6. A silver layer having a 110 nm thicknesswould be vapor coated onto the tie layer. A copper layer (20 nmthickness) would then also be vapor coated onto the silver layer toprotect it from corrosion.

An acrylic resin available under the trade designation V044 AcrylicResin from Arkema, Inc. of Philadelphia, Pa. would be extrusioncompounded with 3 weight percent of a UV-absorber available as TINUVIN1577 from Ciba Specialty Chemicals Corp., and 0.15 weight percent of ahindered amine light stabilizer available as CHIMASSORB 944 from CibaSpecialty Chemicals Corp., and an anhydride-modified ethylene vinylacetate polymer adhesive available as BYNEL E418 from E.I. du Pont deNemours and Co. of Wilmington, Del., and then coextrusion coated as atie layer onto FILM 6 opposite the metallic layer and simultaneouslydirected into a nip under a pressure of 893 kg/m (50 pounds per linealinch) against a casting tool having a reflector finish surface at atemperature of 90 ° F. (32° C.), at a casting line speed of 0.38 metersper second (75 feet per minute) to produce FILM 7. The coextrusioncoated layers would have a total thickness of 150 micrometers (6 mils)with a skin:tie layer thickness ratio of 20:1. The same materials wouldbe coextrusion coated onto the opposing surface of the multilayervisible reflector film.

Prophetic Example 3

A copper layer (150 nm thickness) would be vapor coated onto one side ofFILM 1 as described in Example 1.

An acrylic resin available under the trade designation V044 AcrylicResin from Arkema, Inc. would be extrusion compounded with 3 weightpercent of a UV-absorber available as TINUVIN 1577 from Ciba SpecialtyChemicals Corp., and 0.15 weight percent of a hindered amine lightstabilizer available as CHIMASSORB 944 from Ciba Specialty ChemicalsCorp., and an anhydride-modified ethylene vinyl acetate polymer adhesiveavailable as BYNEL E418 from E.I. du Pont de Nemours and Co.,Wilmington, Del., and then coextrusion coated as a tie layer onto FILM 1opposite the metallic layer and simultaneously directed into a nip undera pressure of 893 kg/m (50 pounds per lineal inch) against a castingtool having a reflector finish surface at a temperature of 90° F. (32°C.), at a casting line speed of 0.38 meters per second (75 feet perminute) resulting in FILM 8. The coextrusion coated layers would have atotal thickness of 150 micrometers (6 mils) with a skin:tie layerthickness ratio of 20:1. The same materials would be coextrusion coatedonto the opposite surface of Film 8.

A scratch resistant coating consisting of a thermally cured siliceouspolymer available as PERMA-NEW 6000 CLEAR HARD COATING SOLUTION fromCalifornia Hardcoating Co. would be coated on top of the acrylic resinskin layer.

Prophetic Example 4

A multilayer reflective reflector would be made with first opticallayers created from polyethylene 2,6-naphthalate (PEN1) and secondoptical layers created from a terpolymer of tetrafluoroethylene,hexafluoropropylene and vinylidene fluoride available as THVP 2030G fromDyneon, LLC, Oakdale, Minn. (THV1). PEN1 and THV1 would be coextrudedthru a multilayer polymer melt manifold to create a multilayer meltstream having 550 alternating first and second optical layers. The layerthickness profile (layer thickness values) of this Vis-IR reflectorwould be adjusted to be approximately a linear profile with the first(thinnest) optical layers adjusted to have about a ¼ wave opticalthickness (index times physical thickness) for 420 nm light andprogressing to the thickest layers which would be adjusted to be about ¼wave thick optical thickness for 2500 nm light. Layer thickness profilesof such films can be adjusted to provide for improved spectralcharacteristics using the axial rod apparatus taught in U.S. Pat. No.6,783,349 (Neavin et al.) combined with layer profile informationobtained with microscopic techniques.

In addition to the first and second optical layers, a pair ofnon-optical layers also comprised of PEN loaded with 2 weight percent ofUV-absorber available as TINUVIN 1577 from Ciba Specialty ChemicalsCorp. would be coextruded as protective skin layers on either side ofthe optical layer stack. This multilayer coextruded melt stream would becast onto a chilled roll at 22 meters per minute creating a multilayercast web approximately 1400 micrometers (55 mils) thick. The multilayercast web would then be heated in a tenter oven at 145° C. for 10 secondsprior to being biaxially oriented to a draw ratio of 3.8×3.8. Theoriented multilayer film would then be further heated to 225° C. for 10seconds to increase crystallinity of the PEN1 layers resulting in FILM9.

The resultant VIS/IR-reflective multiplayer optical film would belaminated to, or coextruded with, a UV-reflective multiplayer opticalfilm made with first optical layers created from PMMA1 available underthe trade designation V044 Acrylic Resin from Arkema, Inc. and secondoptical layers created from a copolymer of tetrafluoroethylene availableas THVP 2030 (THV1) from Dyneon, LLC. PMMA1 and THV1 would be coextrudedthrough a multilayer polymer melt manifold to create a multilayer meltstream having 550 alternating first and second optical layers. The layerthickness profile (layer thickness values) of this UV reflector would beadjusted to be approximately a linear profile with the first (thinnest)optical layers adjusted to have about a ¼ wave optical thickness (indextimes physical thickness) for 340 nm light and progressing to thethickest layers which would be adjusted to be about ¼ wave thick opticalthickness for 420 nm light. Layer thickness profiles of such films canbe adjusted to provide for improved spectral characteristics using theaxial rod apparatus taught in U.S. Pat. No. 6,783,349 (Neavin et al.)combined with layer profile information obtained with microscopictechniques.

In addition to the first and second optical layers, a pair of PMMA1non-optical layers would be coextruded as protective skin layers oneither side of the optical layer stack. These PMMA1 skin layers would beextrusion compounded with 2 weight percent of a UV absorber available asTINUVIN 1577 and 0.15 percent CHIMMASORB 944 from Ciba SpecialtyChemicals Corp. This multilayer coextruded melt stream would be castonto a chilled roll at 22 meters per minute creating a multilayer castweb approximately 300 micrometers (12mils) thick. The multilayer castweb would then be heated in a tenter oven at 135° C. for 10 secondsprior to being biaxially oriented to a draw ratio of 3.8×3.8. Theresultant UV-reflective multilayer optical FILM 10 would then beextrusion hot melt laminated to the VIS/IR-reflective multilayer opticalfilm with a hot melt adhesive available as ADMER SE810 from MitsuiChemicals Americas, Inc. of Rye Brook, N.Y.

A scratch resistant coating consisting of a thermally cured siliceouspolymer available as PERMA-NEW 6000 CLEAR HARD COATING SOLUTION fromCalifornia Hardcoating Co. of Chula Vista, Calif. would be coated on topof the acrylic resin skin layer.

Prophetic Example 5

A broadband reflector film would be made with first optical layerscreated from PET 1 and second optical layers created from OTP 1. PET 1and OTP 1 would be coextruded thru a multilayer polymer melt manifold tocreate a multilayer melt stream having 550 alternating first and secondoptical layers. The layer thickness profile (layer thickness values) ofthis Vis-IR reflector would be adjusted to be approximately a linearprofile with the first (thinnest) optical layers adjusted to have abouta ¼ wave optical thickness (index times physical thickness) for 420 nmlight and progressing to the thickest layers which would be adjusted tobe about ¼ wave thick optical thickness for 1100 nm light. Layerthickness profiles of such films can be adjusted to provide for improvedspectral characteristics using the axial rod apparatus taught in U.S.Pat. No. 6,783,349 (Neavin et al.) combined with layer profileinformation obtained with microscopic techniques.

In addition to the first and second optical layers, a pair ofnon-optical layers consisting of PET1 loaded with 2 weight percent of aUV-absorber available as TINUVIN 1577 from Ciba Specialty ChemicalsCorp. would be coextrusion coated as a protective skin layer on eitherside of the UV-reflective multilayer optical film. This multilayercoextruded melt stream would be cast onto a chilled roll at 22 metersper minute creating a multilayer cast web approximately 1400 micrometers(56 mils) thick. The multilayer cast web would be then heated in atenter oven at 95° C. for 10 seconds prior to being biaxially orientedto a draw ratio of 3.3×3.5. The oriented multilayer film would befurther heated at 225° C. for 10 seconds to increase crystallinity ofthe PET layers resulting in Film 4.

An acrylic resin available under the trade designation V044 AcrylicResin from Arkema, Inc. would be extrusion compounded with 5 weightpercent of a UV-absorber available as TINUVIN 1577 from Ciba SpecialtyChemicals Corp., and 0.15 weight percent of a hindered amine lightstabilizer available as CHIMASSORB 944 from Ciba Specialty ChemicalsCorp., and an anhydride-modified ethylene vinyl acetate polymer adhesiveavailable as BYNEL E418 from E. I. du Pont de Nemours and Co., and thencoextrusion coated as a tie layer onto FILM 4 and simultaneouslydirected into a nip under a pressure of 893 kg/m (50 pounds per linealinch) against a casting tool having a reflector finish surface at atemperature of 90° F. (32° C.), at line speed of 0.38 meters per second(75 feet per minute). The coextrusion coated layers would have a totalthickness of 150 micrometers (6 mils) with skin:tie layer thicknessratio of 20:1. The same materials would be coextrusion coated onto theopposing surface of FILM 4. The resultant film would be laminated to orcoextruded with a UV-reflective multilayer optical film made with firstoptical layers created from PMMA1 and second optical layers created fromTHV1 coextruded thru a multilayer polymer melt manifold to create amultilayer melt stream having 550 alternating first and second opticallayers. The layer thickness profile (layer thickness values) of this UVreflector would be adjusted to be approximately a linear profile withthe first (thinnest) optical layers adjusted to have about a ¼ waveoptical thickness (index times physical thickness) for 340 nm light andprogressing to the thickest layers which would be adjusted to be about ¼wave thick optical thickness for 420 nm light. Layer thickness profilesof such films can be adjusted to provide for improved spectralcharacteristics using the axial rod apparatus taught in U.S. Pat. No.6,783,349 (Neavin et al.) combined with layer profile informationobtained with microscopic techniques.

In addition to the first and second optical layers, a pair ofnon-optical layers comprised of PMMA1 would be coextruded as protectiveskin layers on either side of the optical layer stack. These PMMA1 skinslayers would be extrusion compounded with 2 weight percent of a UVabsorber available as TINUVIN 1577 from Ciba Specialty Chemicals Corp.This multilayer coextruded melt stream would be cast onto a chilled rollat 22 meters per minute creating a multilayer cast web approximately 300micrometers (12 mils) thick. The multilayer cast web would then beheated in a tenter oven at 135° C. for 10 seconds prior to beingbiaxially oriented to a draw ratio of 3.8×3.8.

A scratch resistant coating consisting of a thermally cured siliceouspolymer available as PERMA-NEW 6000 CLEAR HARD COATING SOLUTION fromCalifornia Hardcoating Co. would be coated on top of the acrylic resinskin layer.

Prophetic Example 6

A highly reflective broadband reflector would be made as in PropheticExample 1, but rather than including the sequential silver/copper vaporcoated metal layer, the broadband reflector film would be laminated to asheet of polished anodized aluminum.

Prophetic Example 7

A highly reflective broadband reflector would be made as in Example 2,but rather than including the sequential silver/copper vapor coatedmetal layer, the broadband reflector film would be laminated to a sheetof polished stainless steel.

Prophetic Example 8

A multilayer optical film UV reflective reflector would be made withfirst optical layers created from PMMA1 (an acrylic resin availableunder the trade designation V044 Acrylic Resin from Arkema, Inc.) andsecond optical layers created from a terpolymer of tetrafluoroethylene,hexafluoropropylene and vinylidene fluoride available as THV 2030 fromDyneon, LLC. The PMMA and THV would be coextruded thru a multilayerpolymer melt manifold to create a multilayer melt stream having 550alternating first and second optical layers. The layer thickness profile(layer thickness values) of this UV reflector would be adjusted to beapproximately a linear profile with the first (thinnest) optical layersadjusted to have about a ¼ wave optical thickness (index times physicalthickness) for 340 nm light and progressing to the thickest layers whichwould be adjusted to be about ¼ wave thick optical thickness for 420 nmlight. Layer thickness profiles of such films can be adjusted to providefor improved spectral characteristics using the axial rod apparatustaught in U.S. Pat. No. 6,783,349 (Neavin et al.) combined with layerprofile information obtained with microscopic techniques.

In addition to the first and second optical layers, a pair ofnon-optical layers also comprised of PMMA1 would be coextruded asprotective skin layers on either side of the optical layer stack. ThesePMMA1 skins layers would be extrusion compounded with 2 weight percentof a UV-absorber available as TINUVIN 1577 from Ciba Specialty ChemicalsCorp. This multilayer coextruded melt stream would be cast onto achilled roll at 22 meters per minute creating a multilayer cast webapproximately 300 micrometers (12 mils) thick. The multilayer cast webwould then be heated in a tenter oven at 135° C. for 10 seconds prior tobeing biaxially oriented to a draw ratio of 3.8×3.8 resulting in FILM10.

A titanium dioxide tie layer having a 10 nm thickness would be vaporcoated onto one side of FILM 10.

A silver layer having a 110 nm thickness would be vapor coated onto thetie layer. A copper layer, 20 nm thickness, would then also be vaporcoated onto the silver to protect it from corrosion.

A scratch resistant coating consisting of a thermally cured siliceouspolymer available as PERMA-NEW 6000 CLEAR HARD COATING SOLUTION fromCalifornia Hardcoating Co. would be coated on top of the acrylic resinskin layer.

Prophetic Example 9 Film 11

A UV-VIS reflective multilayer optical film was made with first opticallayers created from polyethylene terephthalate available as EASTAPAK7452 from Eastman Chemical of Kingsport, Tenn., (PET1) and secondoptical layers created from a copolymer of 75 weight percent methylmethacrylate and 25 weight percent ethyl acrylate (available from IneosAcrylics, Inc. of Memphis, Tenn., as PERSPEX CP63) (coPMMA1). The PET1and CoPMMA1 were coextruded through a multilayer polymer melt manifoldto form a stack of 550 optical layers. The layer thickness profile(layer thickness values) of this UV reflector was adjusted to beapproximately a linear profile with the first (thinnest) optical layersadjusted to have about a ¼ wave optical thickness (index times physicalthickness) for 370 nm light and progressing to the thickest layers whichwere adjusted to be about ¼ wave thick optical thickness for 800 nmlight. Layer thickness profiles of such films were adjusted to providefor improved spectral characteristics using the axial rod apparatustaught in U.S. Pat. No. 6,783,349 (Neavin et al.) combined with layerprofile information obtained with microscopic techniques.

In addition to these optical layers, non-optical protective skin layersof PET1 (260 micrometers thickness each) were coextruded on either sideof the optical stack. This multilayer coextruded melt stream was castonto a chilled roll at 5.4 meters per minute creating a multilayer castweb approximately 1100 micrometers (43.9 mils) thick. The multilayercast web was then preheated for about 10 seconds at 95° C. anduniaxially oriented in the machine directon at a draw ratio of 3.3:1.The multilayer cast web was then heated in a tenter oven at 95° C. forabout 10 seconds prior to being uniaxially oriented in the transversedirection to a draw ratio of 3.5:1. The oriented multilayer film wasfurther heated at 225° C. for 10 seconds to increase crystallinity ofthe PET layers. The UV-reflective multilayer optical film (Film 11) wasmeasured with a spectrophotometer (LAMBDA 950 UV/VIS/NIRSPECTROPHOTOMETER from Perkin-Elmer, Inc. of Waltham, Mass.) to have anaverage reflectivity of 96.8 percent over a bandwidth of 370-800 nm.

Film 12

A near infra-red reflective multilayer optical film was made with firstoptical layers created from PET1 and second optical layers created fromcoPMMA1. The PET1 and CoPMMA1 were coextruded through a multilayerpolymer melt manifold to form a stack of 550 optical layers. The layerthickness profile (layer thickness values) of this near infra-redreflector was adjusted to be approximately a linear profile with thefirst (thinnest) optical layers adjusted to have about a ¼ wave opticalthickness (index times physical thickness) for 750 nm light andprogressing to the thickest layers which were adjusted to be about ¼wave thick optical thickness for 1350 nm light. Layer thickness profilesof such films were adjusted to provide for improved spectralcharacteristics using the axial rod apparatus taught in U.S. Pat. No.6,783,349 (Neavin et al.) combined with layer profile informationobtained with microscopic techniques.

In addition to these optical layers, non-optical protective skin layersof PET1 (260 micrometers thickness each) were coextruded on either sideof the optical stack. This multilayer coextruded melt stream was castonto a chilled roll at 3.23 meters per minute creating a multilayer castweb approximately 1800 micrometers (73 mils) thick. The multilayer castweb was then preheated for about 10 seconds at 95° C. and uniaxiallyoriented in the machine directon at a draw ratio of 3.3:1. Themultilayer cast web was then heated in a tenter oven at 95° C. for about10 seconds prior to being uniaxially oriented in the transversedirection to a draw ratio of 3.5:1. The oriented multilayer film wasfurther heated at 225° C. for 10 seconds to increase crystallinity ofthe PET layers. The IR-reflective multilayer optical film (Film 12) wasmeasured with a spectrophotometer (LAMBDA 950 UV/VIS/NIRSPECTROPHOTOMETER from Perkin-Elmer, Inc. of Waltham, Mass.) to have anaverage reflectivity of 96.1 percent over a bandwidth of 750-1350 nm.

A 10 nm thick layer of alumina would then be vacuum vapor coated ontothe exposed surface of FILM 12. A 110 nm thick layer of silver couldthen be vacuum vapor coated onto the alumina layer, and then a 20 nmthick layer of copper was vacuum vapor coated over the silver layer forimproved corrosion resistance.

Average solar reflectivity of this film would be expected to be 97percent over a bandwidth of 300 nm to 2494 nm when measured and thenweight averaged with the Standard Air Mass 1.5 Direct Normal andHemispherical Spectral Solar Irradiance for 37° Sun-Facing TiltedSurface (according to ASTM G 173-03, updated March 2006 and entitled“Standard Tables for Reference Solar Spectral Irradiances: Direct Normaland Hemispherical on 37° Tilted Surface”.

ILLUSTRATIVE EXAMPLE

Film 11 and Film 12 were laminated together using an optically clearadhesive obtained from 3M Company, St. Paul, Minn., as OPTICALLY CLEARLAMINATING ADHESIVE PSA 8171 and then laminated again to a 0.25″ thicksheet of PMMA obtained from Arkema, Inc. under the trade designationPLEXIGLAS VO44. The resulting mirror laminate plates were then attachedto the sides of an 80 watt crystalline silicon photovoltaic module(available under the trade designation SHARP 80W) with added hingeswhich allowed tracking of the sun as shown in FIGS. 8a -c.

Photovoltaic module power output was measured with a handheldvoltage/current meter and calculated by multiplying open circuit voltagewith closed loop current, and then multiplication again by a fill factorof 0.75, with the assumption that the fill factor was not changed by theconcentrating mirrors. Temperature measurements were made both by tapingmultiple thermocouples to the backside of the PV module, and with theuse of an infra-red pyrometer. Power output increases over anon-concentrated solar control photovoltaic module were measured as highas 400% in the mornings when the sun was low in the sky and 40% duringmid-day. Measurements were made for several days in April of 2009 inScandia, Minn., USA which is in a northern latitude and has a temperateclimate. Considerable variability was observed when any clouds or hazeoccurred in the sky so averaging of the data was done. Power measurementresults are shown in Table 1, below. The temperatures of thephotovoltaic modules did not exceed 85° C.

A UV-absorbing layer could be incorporated between Film 11 and Film 12using the method of Prophetic Example 4, above, or on top of Film 11.The trend in the power output observed in Table 1 would not be expectedto substantially change with the addition of the UV-absorbing layer.

TABLE 1 Control Example 11 Time of Day *Power(watts) *Power(watts) %increase 8AM 15.4 76.3 396.1 9AM 36.9 104.9 184.4 10AM 57.8 114.8 98.711AM 77.2 120.8 56.6 12PM 80.4 113.2 40.8 1PM 80.9 110.0 35.9 2PM 74.2108.4 46.1 3PM 68.2 106.6 56.3 4PM 58.0 108.1 86.4 5PM 32.3 105.0 225.66PM 16.9 79.8 372.9 Sum = 549 962.9 75.4 *assumes fill factor of .75

Although this Illustrative Example illustrates the increase in power outof a photovoltaic module, it is expected that a similar relativeincrease in power would occur from a solar thermal panel.

All patents and publications referred to herein are hereby incorporatedby reference in their entirety. Various modifications and alterations ofthis disclosure may be made by those skilled in the art withoutdeparting from the scope and spirit of this disclosure, and it should beunderstood that this disclosure is not to be unduly limited to theillustrative embodiments set forth herein.

What is claimed is:
 1. A broadband reflector comprising: a UV-reflectivemultilayer optical film having a first major surface and comprising aUV-reflective optical layer stack, wherein the UV-reflective opticallayer stack comprises first optical layers and second optical layers,wherein at least a portion of the first optical layers and at least aportion of the second optical layers are in intimate contact and havedifferent refractive indexes, and wherein the UV-reflective opticallayer stack is reflective to UV-light; a VIS/IR-reflective multilayeroptical film comprising a VIS/IR-reflective optical layer stack, whereinthe VIS/IR-reflective optical layer stack comprises third optical layersand fourth optical layers, wherein at least a portion of the thirdoptical layers and at least a portion of the fourth optical layers arein intimate contact and have different refractive indexes, and whereinthe VIS/IR-reflective multilayer optical film is reflective toVIS/IR-light; and a UV-absorbing layer disposed between the first majorsurface of the UV-reflective multilayer optical film and theVIS/IR-reflective multilayer optical film, wherein the UV-absorbinglayer comprises a polymer and a UV-absorber.
 2. The broadband reflectorof claim 1, wherein the third optical layers and fourth optical layersrespectively comprise a polyethylene terephthalate and a THV, apolyethylene terephthalate and an OTP, a PEN and a THV, a PEN and anOTP, a PEN and a PMMA, a polyethylene terephthalate and a coPMMA, a PENand a coPMMA layer pairs, a coPEN and a PMMA layer pairs, a coPEN and anOTP, a coPEN and a THV, a sPS and an OTP, a sPS and a THV, a PMMA and aTHV, a COC and a THV, or an EVA and a THV.
 3. The broadband reflector ofclaim 1, further comprising an adhesive layer disposed on theVIS/IR-reflective multilayer optical film opposite the UV-absorbinglayer.
 4. The broadband reflector claim 1, wherein the first opticallayers and second optical layers respectively comprise a polyethyleneterephthalate and a coPMMA, a sPS and an OTP, a sPS and a THV, a PMMAand a THV, a COC and a THV, or an EVA and a THV.
 5. The broadbandreflector claim 1, wherein the UV-reflective multilayer optical filmfurther comprises a tie layer that comprises the first major surface ofthe UV-reflective multilayer optical film.
 6. The broadband reflector ofclaim 5, wherein the tie layer comprises an inorganic tie layer.
 7. Thebroadband reflector of claim 1, wherein the broadband reflector has anaverage light reflectivity of at least 90 percent over a wavelengthrange of from 300 to 2494 nanometers.
 8. The broadband reflector ofclaim 1, wherein the UV-reflective multilayer optical film furthercomprises a second major surface opposite the first major surface, andwherein the UV-reflective multilayer optical film further comprises anabrasion resistant layer that forms the second major surface of theUV-reflective multilayer optical film.
 9. The broadband reflector ofclaim 8, wherein the abrasion resistant layer comprises: an antisoilingcomponent selected from the group consisting of fluoropolymers, siliconepolymers, titanium dioxide particles, polyhedral oligomericsilsesquioxanes, and combinations thereof.
 10. A concentrated solarpower system comprising: at least one broadband reflector of claim 1capable of being aligned to direct solar radiation onto a hollowreceiver; and a heat transfer fluid partially disposed within the hollowreceiver.
 11. The concentrated solar power system of claim 10, furthercomprising an electrical generator in fluid communication with thehollow receiver.
 12. The concentrated solar power system of claim 10,further comprising a celestial tracking mechanism for the at least onebroadband reflector.
 13. The concentrated solar power system of claim12, wherein the celestial tracking mechanism comprises a louverpivotally mounted adjacent the hollow receiver, wherein the louvercomprises the at least one broadband reflector.
 14. A method ofharnessing solar energy, the method comprising reflecting solarradiation using at least one broadband reflector of claim 1 onto ahollow receiver containing a heat transfer fluid to provide a heatedheat transfer fluid.
 15. The method of claim 14, further comprisingthermally heating at least a portion of a building with heat given offfrom the heated heat transfer fluid.
 16. The method of claim 14, furthercomprising generating electrical power using the heated heat transferfluid.
 17. A method of using the broadband reflector of claim 1, themethod comprising: adhering the broadband reflector to an existing solarreflector adapted for use in a concentrated solar power system.
 18. Asolar collection device comprising at least one solar cell and abroadband reflector according to claim 1 positioned in proximity to theat least one solar cell.